In the field of rock mechanics, the stress characteristics of a medium are often monitored. This is often done by placing a pressure sensor in a borehole defined in the medium. As many data points as possible should be provided to produce an accurate, precise stress reading, and this requirement creates a need for a provision of as many sensors in the borehole as is possible. Furthermore, to be most accurate and precise, a stress picture at each data point should also be provided thereby producing a requirement of versatility for the sensors. Therefore, the sensors must not only be versatile, they must be small and sensitive enough so a great may data points can be measured in the borehole.
Furthermore, since the sensors may be used in a harsh environment, they must also be rugged. Requirements for rugged and durable construction may be contrary to requirements associated with small, sensitive and versatile sensors. Thus, accurate and precise measurement of stress changes in a medium surrounding a borehole in the mining art may be a difficult goal to achieve.
In the past, stress changes have been measured by several types of devices. One type of these prior art devices is exemplified by a coal cell which measures stress changes in only one direction. While such devices may be useful for many purposes, accurate and precise stress change monitoring requires measurement in a plurality of directions at a plurality of locations within the borehole. Therefore, sensors which measure stress changes in but one axial direction are not entirely adequate for such uses.
Another type of sensor used to measure stress changes in a medium surrounding a borehole uses tensioned wires. An example of this type of device is disclosed in U.S. Pat. No. 4,159,641. Pressure changes in the medium changes the tension in the wires, which is then translated into stress changes. While such devices can measure stress changes at a plurality of positions, these devices cannot easily define a stress field or gradient as only a limited number of instruments can be practically placed in a single borehold. Furthermore, such devices may be quite sensitive to temperature or humidity variations or corrosion, and may not be as rugged and durable as is desirable.
Furthermore, many devices may not be usable in all mediums due to instabilities in that medium. For example, devices which use the modulus of elasticity of the medium as the factor monitored would not be usable if the medium is salt because salt creeps at very low pressures (for example, 145 psi) and thus does not have a modulus of elasticity.